For years I've just accepted his presence in the inn criticising the Whiterun guard but who is he? Is he just a random wannabe captain of the guard?

a thalmor sleeper agent.

Commander of the whiterun stormcloak guards when the Stormcloaks take over.

Maybe he's hinting to kill the captain of the guard, then he will step in

Ohhhhh ok. I'm in the middle of my first SC playthrough and haven't visited Whiterun since before the siege.

Not just a wannabe captain of the guards, he becomes captain if the Cloaks take over. Caius takes his place in the Blue Palace basemant and Sinmir becomes captain of the guards. Whiterun sleeps better knowing it is now under Sinmir's protection.

for whatever that's worth.

Bandits will [censored] their pants when they hear dat accent.

Da saghuridahh in Whaitrwan is therhibl. Schaimfu, iz what it izh.

He's probably the smartest resident in Whiterun. He realizes how cruddy the guards are, so he walks around armed and armored. He also stays inside. If the other residents of this town had half his sense, not one of them would get killed in a vampire attack.

Hey, the more unintelligible, the more terrifying the person is going to be.

Leading Whiterun by example, despite not being an official member of the Jarl. This is a true citizen!

A poorly-designed NPC.

He does become an official member if Vignar becomes Jarl.

Why the other members of Whiterun aren't like this beats me. "A master vampire and a legendary dragon!!!!!!!!!!! Come on!!! Is that all you got!?" - Ysolda with an iron dagger getting in the way of my shouts and spells.

Sorry for being the third person to do this. This is just too damn funny.

Does sinmir have role in a thieves guild quest if he becomes captain?

Who is Sinmir you ask?

Sinmir is the prime example of the degeneracy, and degradation of the Nordic race. What do you see? A barbaric heathen who worships false gods, suffers the land with his idleness, while talking of action without merit. A drunk with an ego larger than Morokei, he is a beacon to the other savages that infest this holy land that is Alduin's - savior of all.

And when I break free of Valdar's evil ward, he will be punished according to ancient Atmoran procedure for his blasphemous worship, and blatant laziness.

Yes. I'm surprised he's even able to walk. Even then, its to drink more.

Sinmir never leaves the Inn due to a bug. He is supposed to live in Dragonsreach after the Stormcloaks take over http://www.uesp.net/wiki/Skyrim:Sinmir

He doesn't leave because he's wasted.

He occasionally can be seen on the streets of Whiterun after a takeover.

He probably came out to see the racket and once he saw the mess "Waht, waht hapneppd". Like someone who was still high.

He is Whiterun's villiage idiot.

That might apply to Nazeem as well.